Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http:/​/​www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Information About Administering the Switch

System Time and Date Management

You can manage the system time and date on your switch using automatic configuration, such as the Network Time Protocol (NTP), or manual configuration methods.

System Clock

The basis of the time service is the system clock. This clock runs from the moment the system starts up and keeps track of the date and time.

The system clock can then be set from these sources:

NTP

Manual configuration

The system clock can provide time to these services:

User show commands

Logging and debugging messages

The system clock keeps track of time internally based on Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT). You can configure information about the local time zone and summer time (daylight saving time) so that the time appears correctly for the local time zone.

The system clock keeps track of whether the time is authoritative or not (that is, whether it has been set by a time source considered to be authoritative). If it is not authoritative, the time is available only for display purposes and is not redistributed.

Network Time Protocol

The NTP is designed to time-synchronize a network of devices. NTP runs over User Datagram Protocol (UDP), which runs over IP. NTP is documented in RFC 1305.

An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two devices to within a millisecond of one another.

NTP Stratum

NTP uses the concept of a stratum to describe how many NTP hops away a device is from an authoritative time source. A stratum 1 time server has a radio or atomic clock directly attached, a stratum 2 time server receives its time through NTP from a stratum 1 time server, and so on. A device running NTP automatically chooses as its time source the device with the lowest stratum number with which it communicates through NTP. This strategy effectively builds a self-organizing tree of NTP speakers.

NTP avoids synchronizing to a device whose time might not be accurate by never synchronizing to a device that is not synchronized. NTP also compares the time reported by several devices and does not synchronize to a device whose time is significantly different than the others, even if its stratum is lower.

NTP Associations

The communications between devices running NTP (known as associations) are usually statically configured; each device is given the IP address of all devices with which it should form associations. Accurate timekeeping is possible by exchanging NTP messages between each pair of devices with an association. However, in a LAN environment, NTP can be configured to use IP broadcast messages instead. This alternative reduces configuration complexity because each device can simply be configured to send or receive broadcast messages. However, in that case, information flow is one-way only.

NTP Security

The time kept on a device is a critical resource; you should use the security features of NTP to avoid the accidental or malicious setting of an incorrect time. Two mechanisms are available: an access list-based restriction scheme and an encrypted authentication mechanism.

NTP Implementation

Implementation of NTP does not support stratum 1 service; it is not possible to connect to a radio or atomic clock. We recommend that the time service for your network be derived from the public NTP servers available on the IP Internet.

Figure 1. Typical NTP Network Configuration.
The following figure shows a typical network example using NTP. Switch A is the NTP master, with the Switch B, C, and D configured in NTP server mode, in server association with Switch A. Switch E is configured as an NTP peer to the upstream and downstream switches, Switch B and Switch F, respectively.

If the network is isolated from the Internet, NTP allows a device to act as if it is synchronized through NTP, when in fact it has learned the time by using other means. Other devices then synchronize to that device through NTP.

When multiple sources of time are available, NTP is always considered to be more authoritative. NTP time overrides the time set by any other method.

Several manufacturers include NTP software for their host systems, and a publicly available version for systems running UNIX and its various derivatives is also available. This software allows host systems to be time-synchronized as well.

NTP Version 4

NTP version 4 is implemented on the switch. NTPv4 is an extension of NTP version 3. NTPv4 supports both IPv4 and IPv6 and is backward-compatible with NTPv3.

NTPv4 provides these capabilities:

Support for IPv6.

Improved security compared to NTPv3. The NTPv4 protocol provides a security framework based on public key cryptography and standard X509 certificates.

Automatic calculation of the time-distribution hierarchy for a network. Using specific multicast groups, NTPv4 automatically configures the hierarchy of the servers to achieve the best time accuracy for the lowest bandwidth cost. This feature leverages site-local IPv6 multicast addresses.

System Name and Prompt

You configure the system name on the switch to identify it. By default, the system name and prompt are Switch.

If you have not configured a system prompt, the first 20 characters of the system name are used as the system prompt. A greater-than symbol [>] is appended. The prompt is updated whenever the system name changes.

Stack System Name and Prompt

If you are accessing a stack member through the active switch, you must use the sessionstack-member-number privileged EXEC command. The stack member number range is from 1 through 9. When you use this command, the stack member number is appended to the system prompt. For example, Switch-2# is the prompt in privileged EXEC mode for stack member 2, and the system prompt for the switch stack is Switch.

Default System Name and Prompt Configuration

The default switch system name and prompt is Switch.

DNS

The DNS protocol controls the Domain Name System (DNS), a distributed database with which you can map hostnames to IP addresses. When you configure DNS on your switch, you can substitute the hostname for the IP address with all IP commands, such as ping, telnet, connect, and related Telnet support operations.

IP defines a hierarchical naming scheme that allows a device to be identified by its location or domain. Domain names are pieced together with periods (.) as the delimiting characters. For example, Cisco Systems is a commercial organization that IP identifies by a com domain name, so its domain name is cisco.com. A specific device in this domain, for example, the File Transfer Protocol (FTP) system is identified as ftp.cisco.com.

To keep track of domain names, IP has defined the concept of a domain name server, which holds a cache (or database) of names mapped to IP addresses. To map domain names to IP addresses, you must first identify the hostnames, specify the name server that is present on your network, and enable the DNS.

Default DNS Settings

Table 1 Default DNS Settings

Feature

Default Setting

DNS enable state

Enabled.

DNS default domain name

None configured.

DNS servers

No name server addresses are configured.

Login Banners

You can configure a message-of-the-day (MOTD) and a login banner. The MOTD banner is displayed on all connected terminals at login and is useful for sending messages that affect all network users (such as impending system shutdowns).

The login banner is also displayed on all connected terminals. It appears after the MOTD banner and before the login prompts.

Default Banner Configuration

The MOTD and login banners are not configured.

MAC Address Table

The MAC address table contains address information that the switch uses to forward traffic between ports. All MAC addresses in the address table are associated with one or more ports. The address table includes these types of addresses:

Dynamic address—A source MAC address that the switch learns and then ages when it is not in use.

Static address—A manually entered unicast address that does not age and that is not lost when the switch resets.

The address table lists the destination MAC address, the associated VLAN ID, and port number associated with the address and the type (static or dynamic).

MAC Address Table Creation

With multiple MAC addresses supported on all ports, you can connect any port on the switch to other network devices. The switch provides dynamic addressing by learning the source address of packets it receives on each port and adding the address and its associated port number to the address table. As devices are added or removed from the network, the switch updates the address table, adding new dynamic addresses and aging out those that are not in use.

The aging interval is globally configured. However, the switch maintains an address table for each VLAN, and STP can accelerate the aging interval on a per-VLAN basis.

The switch sends packets between any combination of ports, based on the destination address of the received packet. Using the MAC address table, the switch forwards the packet only to the port associated with the destination address. If the destination address is on the port that sent the packet, the packet is filtered and not forwarded. The switch always uses the store-and-forward method: complete packets are stored and checked for errors before transmission.

MAC Addresses and VLANs

All addresses are associated with a VLAN. An address can exist in more than one VLAN and have different destinations in each. Unicast addresses, for example, could be forwarded to port 1 in VLAN 1 and ports 9, 10, and 1 in VLAN 5.

Each VLAN maintains its own logical address table. A known address in one VLAN is unknown in another until it is learned or statically associated with a port in the other VLAN.

When private VLANs are configured, address learning depends on the type of MAC address:

Dynamic MAC addresses learned in one VLAN of a private VLAN are replicated in the associated VLANs. For example, a MAC address learned in a private-VLAN secondary VLAN is replicated in the primary VLAN.

Static MAC addresses configured in a primary or secondary VLAN are not replicated in the associated VLANs. When you configure a static MAC address in a private VLAN primary or secondary VLAN, you should also configure the same static MAC address in all associated VLANs.

MAC Addresses and Switch Stacks

The MAC address tables on all stack members are synchronized. At any given time, each stack member has the same copy of the address tables for each VLAN. When an address ages out, the address is removed from the address tables on all stack members. When a switch joins a switch stack, that switch receives the addresses for each VLAN learned on the other stack members. When a stack member leaves the switch stack, the remaining stack members age out or remove all addresses learned by the former stack member.

Default MAC Address Table Settings

The following table shows the default settings for the MAC address table.

Table 2

Feature

Default Setting

Aging time

300 seconds

Dynamic addresses

Automatically learned

Static addresses

None configured

ARP Table Management

To communicate with a device (over Ethernet, for example), the software first must learn the 48-bit MAC address or the local data link address of that device. The process of learning the local data link address from an IP address is called address resolution.

The Address Resolution Protocol (ARP) associates a host IP address with the corresponding media or MAC addresses and the VLAN ID. Using an IP address, ARP finds the associated MAC address. When a MAC address is found, the IP-MAC address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP). By default, standard Ethernet-style ARP encapsulation (represented by the arpa keyword) is enabled on the IP interface.

ARP entries added manually to the table do not age and must be manually removed.

How to Administer the Switch

Configuring the Time and Date Manually

If no other source of time is available, you can manually configure the time and date after the system is restarted. The time remains accurate until the next system restart. We recommend that you use manual configuration only as a last resort. If you have an outside source to which the switch can synchronize, you do not need to manually set the system clock.

Note

You must reconfigure this setting if you have manually configured the system clock before the active switch fails and a different stack member assumes the role of active switch.

Configures summer time to start and end on the specified days every year. All times are relative to the local time zone. The start time is relative to standard time.

The end time is relative to summer time. Summer time is disabled by default. If you specify clock summer-timezonerecurring without parameters, the summer time rules default to the United States rules.

If the starting month is after the ending month, the system assumes that you are in the southern hemisphere.

zone—Specifies the name of the time zone (for example, PDT) to be displayed when summer time is in effect.

(Optional) week— Specifies the week of the month (1 to 4, first, or last).

(Optional) day—Specifies the day of the week (Sunday, Monday...).

(Optional) month—Specifies the month (January, February...).

(Optional) hh:mm—Specifies the time (24-hour format) in hours and minutes.

(Optional) offset—Specifies the number of minutes to add during summer time. The default is 60.

Step 4

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Configuring a System Name

SUMMARY STEPS

1.configureterminal

2.hostnamename

3.end

DETAILED STEPS

Command or Action

Purpose

Step 1

configureterminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 2

hostnamename

Example:

Switch(config)# hostname B18-lab

Configures a system name. When you set the system name, it is also used as the system prompt.

The default setting is Switch.

The name must follow the rules for ARPANET hostnames. They must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphens. Names can be up to 63 characters.

Step 3

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Setting Up DNS

If you use the switch IP address as its hostname, the IP address is used and no DNS query occurs. If you configure a hostname that contains no periods (.), a period followed by the default domain name is appended to the hostname before the DNS query is made to map the name to an IP address. The default domain name is the value set by the ip domain-name global configuration command. If there is a period (.) in the hostname, the Cisco IOS software looks up the IP address without appending any default domain name to the hostname.

SUMMARY STEPS

1.configureterminal

2.ip domain-namename

3.ip name-serverserver-address1 [server-address2 ... server-address6]

4.ip domain-lookup

5.end

DETAILED STEPS

Command or Action

Purpose

Step 1

configureterminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 2

ip domain-namename

Example:

Switch(config)# ip domain-name name

Defines a default domain name that the software uses to complete unqualified hostnames (names without a dotted-decimal domain name).

Do not include the initial period that separates an unqualified name from the domain name.

At boot time, no domain name is configured; however, if the switch configuration comes from a BOOTP or Dynamic Host Configuration Protocol (DHCP) server, then the default domain name might be set by the BOOTP or DHCP server (if the servers were configured with this information).

Specifies the address of one or more name servers to use for name and address resolution.

You can specify up to six name servers. Separate each server address with a space. The first server specified is the primary server. The switch sends DNS queries to the primary server first. If that query fails, the backup servers are queried.

If your network devices require connectivity with devices in networks for which you do not control name assignment, you can dynamically assign device names that uniquely identify your devices by using the global Internet naming scheme (DNS).

Step 5

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Configuring a Message-of-the-Day Login Banner

You can create a single or multiline message banner that appears on the screen when someone logs in to the switch

SUMMARY STEPS

1.configure terminal

2.banner motdcmessage c

3.end

DETAILED STEPS

Command or Action

Purpose

Step 1

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 2

banner motdcmessage c

Example:

Switch(config)# banner motd #
This is a secure site. Only
authorized users are allowed.
For access, contact technical
support.
#

Specifies the message of the day.

c—Enters the delimiting character of your choice, for example, a pound sign (#), and press the Return key. The delimiting character signifies the beginning and end of the banner text. Characters after the ending delimiter are discarded.

message—Enters a banner message up to 255 characters. You cannot use the delimiting character in the message.

Step 3

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Configuring a Login Banner

You can configure a login banner to be displayed on all connected terminals. This banner appears after the MOTD banner and before the login prompt.

c— Enters the delimiting character of your choice, for example, a pound sign (#), and press the Return key. The delimiting character signifies the beginning and end of the banner text. Characters after the ending delimiter are discarded.

message—Enters a login message up to 255 characters. You cannot use the delimiting character in the message.

version—Specifies the SNMP version to support. Version 1, the default, is not available with informs.

community-string—Specifies the string to send with the notification operation. Though you can set this string by using the snmp-server host command, we recommend that you define this string by using the snmp-server community command before using the snmp-server host command.

notification-type—Uses the mac-notification keyword.

Step 3

snmp-server enable traps mac-notification change

Example:

Switch(config)# snmp-server enable traps mac-notification change

Enables the switch to send MAC address change notification traps to the NMS.

version—Specifies the SNMP version to support. Version 1, the default, is not available with informs.

community-string—Specifies the string to send with the notification operation. Though you can set this string by using the snmp-server host command, we recommend that you define this string by using the snmp-server community command before using the snmp-server host command.

notification-type—Uses the mac-notification keyword.

Step 3

snmp-server enable traps mac-notification move

Example:

Switch(config)# snmp-server enable traps mac-notification move

Enables the switch to send MAC address move notification traps to the NMS.

Step 4

mac address-table notification mac-move

Example:

Switch(config)# mac address-table notification mac-move

Enables the MAC address move notification feature.

Step 5

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Configuring MAC Threshold Notification Traps

When you configure MAC threshold notification, an SNMP notification is generated and sent to the network management system when a MAC address table threshold limit is reached or exceeded.

version—Specifies the SNMP version to support. Version 1, the default, is not available with informs.

community-string—Specifies the string to send with the notification operation. You can set this string by using the snmp-server host command, but we recommend that you define this string by using the snmp-server community command before using the snmp-server host command.

mac-addr—Specifies the destination MAC unicast address to add to the address table. Packets with this destination address received in the specified VLAN are forwarded to the specified interface.

vlan-id—Specifies the VLAN for which the packet with the specified MAC address is received. Valid VLAN IDs are 1 to 4094.

interface-id—Specifies the interface to which the received packet is forwarded. Valid interfaces include physical ports or port channels. For static multicast addresses, you can enter multiple interface IDs. For static unicast addresses, you can enter only one interface at a time, but you can enter the command multiple times with the same MAC address and VLAN ID.

Step 3

end

Example:

Switch(config)# end

Returns to privileged EXEC mode. Alternatively, you can also press Ctrl-z to exit global configuration mode.

Configuring Unicast MAC Address Filtering

SUMMARY STEPS

1.configure terminal

2.mac address-table staticmac-addrvlanvlan-iddrop

3.end

DETAILED STEPS

Command or Action

Purpose

Step 1

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 2

mac address-table staticmac-addrvlanvlan-iddrop

Example:

Switch(config)# mac address-table static c2f3.220a.12f4 vlan 4 drop

Enables unicast MAC address filtering and configure the switch to drop a packet with the specified source or destination unicast static address.

mac-addr—Specifies a source or destination unicast MAC address. Packets with this MAC address are dropped.

vlan-id—Specifies the VLAN for which the packet with the specified MAC address is received. Valid VLAN IDs are 1 to 4094.

Step 3

end

Example:

Switch(config)# end

Returns to privileged EXEC mode.

Monitoring and Maintaining Administration of the Switch

Command

Purpose

clear mac address-table dynamic

Removes all dynamic entries.

clear mac address-table dynamic addressmac-address

Removes a specific MAC address.

clear mac address-table dynamic interface interface-id

Removes all addresses on the specified physical port or port channel.

clear mac address-table dynamic vlan vlan-id

Removes all addresses on a specified VLAN.

show clock [detail]

Displays the time and date configuration.

show ip igmp snooping groups

Displays the Layer 2 multicast entries for all VLANs or the specified VLAN.

show mac address-table addressmac-address

Displays MAC address table information for the specified MAC address.

show mac address-table aging-time

Displays the aging time in all VLANs or the specified VLAN.

show mac address-table count

Displays the number of addresses present in all VLANs or the specified VLAN.

show mac address-table dynamic

Displays only dynamic MAC address table entries.

show mac address-table interfaceinterface-name

Displays the MAC address table information for the specified interface.

show mac address-table move update

Displays the MAC address table move update information.

show mac address-table multicast

Displays a list of multicast MAC addresses.

show mac address-table notification {change | mac-move | threshold}

Displays the MAC notification parameters and history table.

show mac address-table secure

Displays the secure MAC addresses.

show mac address-table static

Displays only static MAC address table entries.

show mac address-table vlanvlan-id

Displays the MAC address table information for the specified VLAN.

Configuration Examples for Switch Administration

Setting the System Clock: Example

This example shows how to manually set the system clock to 1:32 p.m. on July 23, 2001:

Switch# clock set 13:32:00 23 July 2001

Configuring Summer Time: Examples

This example (for daylight savings time) shows how to specify that summer time starts on the first Sunday in April at 02:00 and ends on the last Sunday in October at 02:00:

Configuring MAC Address Change Notification Traps: Example

This example shows how to specify 172.20.10.10 as the NMS, enable MAC address notification traps to the NMS, enable the MAC address-change notification feature, set the interval time to 123 seconds, set the history-size to 100 entries, and enable traps whenever a MAC address is added on the specified port:

Adding the Static Address to the MAC Address Table: Example

This example shows how to add the static address c2f3.220a.12f4 to the MAC address table. When a packet is received in VLAN 4 with this MAC address as its destination address, the packet is forwarded to the specified port:

Configuring Unicast MAC Address Filtering: Example

This example shows how to enable unicast MAC address filtering and how to configure drop packets that have a source or destination address of c2f3.220a.12f4. When a packet is received in VLAN 4 with this MAC address as its source or destination, the packet is dropped:

Technical Assistance

Description

Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.